1. Field of the Invention
The invention pertains to a transmission belt comprising a cord with at least two fused yarns, to a method of manufacturing the cord, and to a method of manufacturing the transmission belt.
2. Discussion of Related Art
Cords for reinforcing rubber articles are known in the art. A cord for that purpose comprising at least one high-modulus yarn and at least one low-modulus yarn is disclosed in WO 97/06297. The yarns of these cords may be twisted together and can be dipped with a rubber adhesive material. The low-modulus yarn is primarily added as a process aid to enable high-modulus yarns to be used in mould curing processes. By this method transmission belts can be produced; however, during the processing of such belts the mechanical properties of the cord tend to deteriorate.
High bundle cohesion is essential to avoid fraying when the belts get their final shape as they are cut out of a rubber composite slab. In order to produce a clean cut, all the filaments in the yam bundle have to be secured firmly together in the cutting plane. If they are not held in place, the applied cutting force can move filaments out of the cutting plane, causing filaments to be cut at different lengths (the effect called “fraying”). In order to meet the quality standards set by the belt industry, fraying must be kept to an absolute minimum, not for optical reasons only but also to prevent a possible failure initiation. For that reason both aramid and polyester cords are usually pre-dipped with a solvent-based MDI (diphenylmethane-4,4-diisocyanate) pre-dip to obtain high filament coherence. The pre-dipping with MDI results in a rather stiff cord with excellent cutting behavior, though at the cost of poor strength efficiency after the dipping process (10 to 20% strength loss compared to standard “soft-dipping”). Moreover, it was found that stiff-dipped p-aramid cords suffer from severe strength loss after handling and vulcanization. This strength loss is proportional to the stiffness (i.e. the degree of impregnation) and is presumably induced by kink bands while buckling the stiff aramid cords. This phenomenon resulting in loss of strength while handling or processing stiff-dipped cords is called “handling resistance” or “handleability”.